The overall purpose of this research is to obtain a better understanding of the ultrastructure, growth processes and functions of the placental membranes (chorioallantoic and yolk sac) of the golden hamster and closely related myomorph rodent species (rat, mouse, gerbil). Four pertinent current problems in rodent placentology have been selected for specific study. One of these is the permeability of the developing placental membranes to molecular substances during the early, teratogenically-sensitive period of gestation. Electron dense molecular tracer particles (horseradish peroxidase and ferritin) will be used in conjunction with electron microscopy to investigate this problem. A second problem concerns the type and distribution of plasma membrane specializations which structurally and functionally unite the component cells and/or syncytial layers of the choricallantoic and yolk sac placentas of rodents. The approach to this problem will be the recently developed techniques for examining carbon-platinum replicas of freeze-fractured and etched tissues by electron microscopy. Another problem is the determination in the hamster of the cellular and/or syncytial proliferations responsible for the expansive growth of the labyrinthine part of the chorioallantoic placenta following its initial establishment. High resolution cellular radioautography combined with H3-thyridine tissue labeling will be used in this investigation. The last problem has to do with the ultrastructure, vascular wall interactions and functional significance of the trophoblastic giant cells which invade the arterial channels of the uterus during pregnancy in rodents. This problem will be addressed by correlative light and electron microscopic study.